1. Field of the Invention
This invention relates generally to an apparatus and method for sensing physical phenomena and particularly to an optical temperature and position sensing system for detecting temperatures at various locations while measuring position of one or more displaceable elements. Still more particularly, the invention relates to a high accuracy light radar, fiber optic temperature sensing system for use on an aircraft in order to measure temperatures at various locations on the aircraft while simultaneously measuring the positions of various moving parts of the aircraft at high rates and with short lag times.
2. Background of the Related Art
Traditionally, electrical sensors are used to measure the temperature at various locations on an aircraft. Results of all of these measurements are then fed back to a system flight controller which processes this information and outputs appropriate commands to control the actuators.
While this is taking place, position measurements are also being made. A typical actuator has a rod secured within an outer casing. Depending on the actuator, the rod can move back and forth a maximum distance of a few millimeters to over 50 cm. This maximum distance is often referred to as a stroke. A sensor head associated with the actuator sends a position signal representing the position of the actuator rod to a processor that calculates a position measurement. Position measurements of the rod must be fed to the flight controller at rates up to several hundred Hz, with a lag time less than 0.5 ms, and accuracies of a few hundred micrometers.
Fiber optic sensing systems offer numerous advantages over conventional electrical sensing systems for measuring temperatures and positions. First, they are small and lightweight. In addition, they can be made immune from electromagnetic interference (EMI) which can occur near power lines, and electromagnetic pulses (EMP) which can occur in the event of a nuclear explosion. EMI/EMP immunity is an especially important advantage for new generation aircraft which have skins made largely of composite (non-metallic, non-shielding) material. Without heavy, bulky and expensive shielding of conventional electrical sensors and control lines, these next generation aircraft can not be safely flown in areas of severe EMI/EMP. Therefore, "fly-by-light" systems or fiber optic position sensing systems have the potential to replace "fly-by-wire" systems in future aircraft.
One method for fiber-optic temperature sensing is an interferometric optical system as shown in FIG. 1a. There, a laser 700 is coupled to an optical fiber 710 which guides laser radiation from laser 700 to sensor head 720. Sensor head 720 has a fiber optic coupler 730A which provides two optical paths (fibers), namely, a sensing arm 740 and a reference arm 750. Part of the laser radiation travels through sensing arm 740 while another part of the radiation travels through reference arm 750. Sensing arm 740 is wrapped around an object 744 which expands or contracts and in turn causes the optical path length of sensing arm 740 to increase or decrease depending on the temperature at sensor head 720. Laser radiation from sensing arm 740 and reference arm 750 is recombined at coupler 730B and then travels along return fiber 750 to detector 760. Since laser radiation from laser 700 experiences two different delays depending on the difference in relative optical path lengths of sensing arm 740 and reference arm 750, interference fringes are produced at photodetector 760. A fringe counter 770 such as a zero counter then counts the number of fringes at photodetector 760 and this number of fringes is used to estimate temperature changes.
This approach to measuring temperature has numerous disadvantages. First, it can only measure relative temperature variations, i.e., it cannot distinguish between increasing or decreasing temperature. Also, it requires two fibers--transmit fiber 710 and return fiber 750. In addition, it requires single mode, polarization preserving fiber and a coherent laser source. Further, it cannot be multiplexed, e.g., a single photodetector cannot be used to measure temperatures at multiple locations. Also, it cannot receive and process optical signals from both temperature sensor heads and position sensor heads.
Just as other fiber optic temperature sensing systems cannot multiplex temperature sensors with position sensors, other fiber optic position sensing systems cannot multiplex position sensors with temperature sensors. For example, some fiber optic position sensing systems use digital or optical encoding techniques in order to vary the amplitude of an incident optical signal as a surface is moved. However, multiplexing position sensor heads alone in these types of sensor systems is costly and complex, resulting in a heavy, voluminous sensing system which still cannot simultaneously multiplex both temperature and position sensor heads.
Another type of fiber optic position sensor system sometimes called an optical time domain reflectometer (OTDR) uses a pulsed optical source. In particular, OTDRs measure distances to in-line fiber reflectors by estimating the round trip transit time of a light pulse from the pulsed optical source to the in-line fiber reflector and back to a detector. Both the measurement accuracy and estimation times are fundamentally limited by the amplitude and width of the light pulse. It is also difficult to multiplex only position sensor heads in OTDR systems.
Yet another type of fiber optic position sensor system is a coherent optical frequency domain reflectometer (COFDR). COFDRs use coherent frequency modulated (FM) optical radiation. However, optical sources used in the COFDR must have narrow line widths and therefore tend to have low output power and low reliability. Also, as with the interferometer approach to measuring temperature, COFDRs require single mode, polarization preserving fibers in order to coherently optically mix returned optical signals with an optical local oscillator signal. Consequently, COFDRs are difficult to install and maintain. In addition, each temperature sensor heads requires two fibers, one serving as a temperature sensing arm and the other as a reference arm even further complicating the system.